Implement Graphviz, Figlet and Cowsay as Remarkup interpreter blocks

Summary: Fixes T3274. Fixes T3964. Ref T3591.

Test Plan: {F70928}

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T3274, T3964, T3591

Differential Revision: https://secure.phabricator.com/D7332